The catalytic epoxidation of olefins over silver-based catalysts, yielding the corresponding olefin oxide, has been known for a long time. Conventional silver-based catalysts have provided the olefin oxides with notoriously low selectivity. For example, when using conventional catalysts in the epoxidation of ethylene, the selectivity towards ethylene oxide, expressed as a fraction of the ethylene converted, does not reach values above the 6/7 or 85.7 mole-% limit. Therefore, this limit has long been considered to be the theoretically maximal selectivity of this reaction, based on the stoichiometry of the reaction equation7 C2H4+6 O2=>6 C2H4O+2 CO2+2 H2O,cf. Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd ed., Vol. 9, 1980, p. 445.
The selectivity determines to a large extent the economical attractiveness of an epoxidation process. For example, an improvement in the selectivity of the epoxidation process can reduce the operating costs of a large-scale ethylene oxide plant by using less olefin to produce the same amount of olefin oxide or increase revenue by producing more olefin oxide from the same amount of olefin.
The olefin oxide produced by the epoxidation process may be reacted with water to form a 1,2-diol, with carbon dioxide to form a 1,2-carbonate, with an alcohol to form a 1,2-diol ether, or with an amine to form an alkanolamine. Thus, 1,2-diols, 1,2-carbonates, 1,2-diol ethers, and alkanolamines may be produced in a multi-step process initially comprising olefin epoxidation and then the conversion of the formed olefin oxide with water, carbon dioxide, an alcohol, or an amine. Any improvement in the epoxidation process can also lead to an improved process for the production of a 1,2-diol, a 1,2-diol ether or an alkanolamine.
Modern silver-based epoxidation catalysts are highly selective towards olefin oxide production. When using the modern catalysts in the epoxidation of ethylene the selectivity towards ethylene oxide can reach values above the 6/7 or 85.7 mole-% limit referred to. An example of such highly selective catalysts is a catalyst comprising silver and a rhenium promoter, cf. for example U.S. Pat. Nos. 4,761,394 and 4,766,105.
A reaction modifier, for example an organic halide, may be added to the feed in an epoxidation process for increasing the selectivity of a highly selective catalyst (cf. for example EP-A-352850, U.S. Pat. Nos. 4,761,394 and 4,766,105, which are herein incorporated by reference). The reaction modifier suppresses the undesirable oxidation of olefin or olefin oxide to carbon dioxide and water, relative to the desired formation of olefin oxide, by a so-far unexplained mechanism. EP-A-352850 teaches that there is an optimum in the selectivity as a function of the quantity of organic halide in the feed, at a constant oxygen conversion level and given set of reaction conditions.
U.S. Pat. No. 7,102,022 B2 relates to the start-up of an epoxidation process wherein a highly selective catalyst is employed. In this document, there is disclosed an improved start-up procedure wherein the highly selective catalyst is subjected to a heat treatment wherein the catalyst is contacted with a feed comprising oxygen at a temperature above the normal operating temperature of the highly selective catalyst (i.e., above 260° C.).
US-A1-2004/0049061 relates to a method of improving the selectivity of a highly selective catalyst having a low silver density. In this document, there is disclosed a method wherein the highly selective catalyst is subjected to a heat treatment which comprises contacting the catalyst with a feed comprising oxygen at a temperature above the normal operating temperature of the highly selective catalyst (i.e., above 250° C.).
U.S. Pat. No. 4,874,879 relates to the start-up of an epoxidation process employing a highly selective catalyst. In this document, there is disclosed an improved start-up procedure wherein the highly selective catalyst is first contacted with a feed containing an organic chloride moderator and ethylene, and optionally a ballast gas, at a temperature below the normal operating temperature of the catalyst.
EP-B1-1532125 relates to the start-up of an epoxidation process wherein a highly selective catalyst is employed. In this document, there is disclosed an improved start-up procedure wherein the highly selective catalyst is first subjected to a pre-soak phase in the presence of a feed containing an organic halide and is then subjected to a stripping phase in the presence of a feed which is free of the organic halide or may comprise the organic halide in a low quantity. The stripping phase is taught to continue for a period of more than 16 hours up to 200 hours.
It goes without saying that there is an economical incentive to shorten the start-up period and make the catalyst operate at a high selectivity with a minimum delay.